Device for determining deterioration of a catalytic converter for an engine

Abstract

The device according to the present invention controls air-fuel ratio of the exhaust gas flowing into the catalytic converter in such a manner that the exhaust gas flowing into the catalytic converter fluctuates around a center value on the lean air-fuel ratio side compared to the stoichiometric air-fuel ratio when it performs the determination of deterioration of the catalytic converter. In this condition, the device calculates the rich gas inflow amount which is the amount of HC and CO in the exhaust gas flowing into the catalytic converter and the rich gas outflow amount which is the amount of HC and CO in the exhaust gas flowing out from the catalytic converter. The device, then, determines the degree of deterioration of the catalytic converter based on the calculated values of the rich gas inflow amount and the rich gas outflow amount. Since the difference between the rich gas inflow amount and the rich gas outflow amount represents the amount of HC and CO oxidized by the catalytic converter, the deterioration of the catalytic converter can be determined accurately by comparing the rich gas inflow amount and the rich gas outflow amount.

I claim: 
     
       1. A device for determining deterioration of a catalytic converter having an O 2  storage capability and disposed in an exhaust gas passage of an engine comprising: an upstream air-fuel ratio sensor disposed in an exhaust gas passage of an engine upstream of a catalytic converter for detecting an air-fuel ratio of an exhaust gas upstream of the catalytic converter;   a downstream air-fuel ratio sensor disposed in the exhaust gas passage of the engine downstream of the catalytic converter for detecting the air-fuel ratio of the exhaust gas downstream of the catalytic converter;   air-fuel ratio feedback control means for controlling the air-fuel ratio of the exhaust gas flowing into the catalytic converter in such a manner that the air-fuel ratio of the exhaust gas flowing into the catalytic converter changes between a rich air-fuel ratio and a lean air-fuel ratio compared to a stoichiometric air-fuel ratio alternately;   rich gas inflow amount calculating means for calculating the amount of HC and CO in the exhaust gas flowing into the catalytic converter based on the output of the upstream air-fuel ratio sensor when the air-fuel ratio of the exhaust gas is controlled by said air-fuel ratio feedback control means;   rich gas outflow amount calculating means for calculating the amount of HC and CO in the exhaust gas flowing out from the catalytic converter based on the output of the downstream air-fuel ratio sensor when the air-fuel ratio of the exhaust gas is controlled by said air-fuel ratio feedback control means; and   determining means for determining a degree of deterioration of the catalytic converter based on the inflow amount of HC and CO calculated by said rich gas inflow amount calculating means and the outflow amount of HC and CO calculated by the outflow rich gas calculating means.   
     
     
       2. A device according to claim 1, wherein said rich gas inflow amount calculating means calculates said inflow amount of HC and CO based on the output of the upstream air-fuel ratio sensor during a rich gas inflow amount calculating period which starts when the air-fuel ratio detected by the upstream air-fuel ratio changes from lean to rich compared to the stoichiometric air-fuel ratio and terminates when the air-fuel ratio detected by the upstream air-fuel ratio changes from lean to rich compared to the stoichiometric air-fuel ratio, and said rich gas outflow amount calculating means calculates said outflow amount of HC and CO based on the output of the downstream air-fuel ratio sensor during a rich gas outflow amount calculating period which starts when the air-fuel ratio detected by the upstream air-fuel ratio sensor changes from lean to rich compared to the stoichiometric air-fuel ratio and terminates when a predetermined time has elapsed after the air-fuel ratio detected by the upstream air-fuel ratio changed from rich to lean compared to the stoichiometric air-fuel ratio. 
     
     
       3. A device according to claim 2, wherein said rich gas inflow amount calculating means calculates said inflow amount of HC and CO by integrating the difference between the output of the upstream air-fuel ratio sensor and the output of the upstream air-fuel ratio sensor corresponding to the stoichiometric air-fuel ratio during said rich gas inflow amount calculating period, and said outflow rich gas calculating means calculates said outflow amount of HC and CO by integrating the difference between the output of the downstream air-fuel ratio sensor and the output of the downstream sensor at the beginning of said rich gas outflow amount calculating period during said rich gas outflow amount calculating period. 
     
     
       4. A device according to claim 1, wherein said air-fuel ratio feedback control means further controls the air-fuel ratio of the exhaust gas flowing into the catalytic converter so that the air-fuel ratio of the exhaust gas fluctuates around a center value on the lean air-fuel ratio side compared to the stoichiometric air-fuel ratio. 
     
     
       5. A device according to claim 4, wherein said rich gas inflow amount calculating means calculates said inflow amount of HC and CO based on the output of the upstream air-fuel ratio sensor during a rich gas inflow amount calculating period which starts when the air-fuel ratio detected by the upstream air-fuel ratio changes from lean to rich compared to the stoichiometric air-fuel ratio and terminates when the air-fuel ratio detected by the upstream air-fuel ratio changes from lean to rich compared to the stoichiometric air-fuel ratio, and said rich gas outflow amount calculating means calculates said outflow amount of HC and CO based on the output of the downstream air-fuel ratio sensor during a rich gas outflow amount calculating period which starts when the air-fuel ratio detected by the upstream air-fuel ratio sensor changes from lean to rich compared to the stoichiometric air-fuel ratio and terminates when a predetermined time has elapsed after the air-fuel ratio detected by the upstream air-fuel ratio changes from rich to lean compared to the stoichiometric air-fuel ratio. 
     
     
       6. A device according to claim 4, further comprises saturation determining means for determining whether the catalytic converter has absorbed oxygen to its maximum O 2  storage capacity during the period in which the air-fuel ratio of the exhaust gas flowing into the catalytic converter is on a lean side compared to the stoichiometric air-fuel ratio and prohibiting means for prohibiting the determining means from determining the degree of deterioration of the catalytic converter when the saturation determining means determines that the catalytic converter has not absorbed oxygen to its maximum O 2  storage capacity. 
     
     
       7. A device according to claim 6, wherein said rich gas inflow amount calculating means calculates said inflow amount of HC and CO based on the output of the upstream air-fuel ratio sensor during a rich gas inflow amount calculating period which starts when the air-fuel ratio detected by the upstream air-fuel ratio changes from lean to rich compared to the stoichiometric air-fuel ratio and terminates when the air-fuel ratio detected by the upstream air-fuel ratio changes from lean to rich compared to the stoichiometric air-fuel ratio, and said rich gas outflow amount calculating means calculates said outflow amount of HC and CO based on the output of the downstream air-fuel ratio sensor during a rich gas outflow amount calculating period which starts when the air-fuel ratio detected by the upstream air-fuel ratio sensor changes from lean to rich compared to the stoichiometric air-fuel ratio and terminates when a predetermined time has elapsed after the air-fuel ratio detected by the upstream air-fuel ratio changed from rich to lean compared to the stoichiometric air-fuel ratio. 
     
     
       8. A device according to claim 7, wherein said saturation determining means determines whether the catalytic converter has absorbed oxygen to its maximum O 2  storage capacity based on the output of the upstream air-fuel ratio sensor during a saturation determining period which starts when the air-fuel ratio detected by the upstream air-fuel ratio sensor changes from rich to lean compared to the stoichiometric air-fuel ratio and terminates when the air-fuel ratio detected by the upstream air-fuel ratio sensor changes from lean to rich compared to the stoichiometric air-fuel ratio. 
     
     
       9. A device according to claim 6, wherein said saturation determining means determines whether the catalytic converter has absorbed oxygen to its maximum O 2  storage capacity based on the output of the upstream air-fuel ratio sensor during a saturation determining period which starts when the air-fuel ratio detected by the upstream air-fuel ratio sensor changes from rich to lean compared to the stoichiometric air-fuel ratio and terminates when the air-fuel ratio detected by the upstream air-fuel ratio sensor changes from lean to rich compared to the stoichiometric air-fuel ratio. 
     
     
       10. A device according to claim 9, wherein said saturation determining means integrates the difference between the output of the upstream air-fuel ratio sensor and the output of the upstream air-fuel ratio sensor corresponding to the stoichiometric air-fuel ratio during said saturation determining period, and determines that the catalytic converter has not absorbed oxygen to its maximum O 2  storage capacity when said integrated value becomes larger than a predetermined value.